Thanks for the additional information. It sounds like the black on her gums is pigment and nothing to worry about. The darker gums may indicate dehydration or possibly normal for her if close to the gum line; gums are usually a pale pink color.
If the source of the blood that you are seeing is, indeed, her vagina, then there are several possible explanations, especially if you've ruled out an estrus cycle:
1. She could have an infection in her uterus which is otherwise known as a Pyometra. When a vaginal discharge is present in an unspayed female and isn't at the time that we expect, then this condition is always suspected. It can be a somewhat serious situation since the kidneys can be damaged...sometimes irreversibly so.
X-rays are usually taken and can be diagnostic although sometimes an ultrasound is needed. An analysis of the discharge may also be helpful in determining if this is the problem.
Oral antibiotics are not terribly effective for this condition; we usually recommend surgery to remove the uterus. I imagine this seems pretty daunting for a dog this age but many, many older females are spayed all the time and most do very well.
2. A bladder infection is possibly but usually there is blood when a female urinates...not always, though. It's possible that there would be overflow which could be pretty bloody.
3. Unfortunately, bladder cancer can be seen in a dog this age which could cause these symptoms. It would take an ultrasound to diagnose and unfortunately, the prognosis isn't very good.
4. A vaginal polyp or possibly cancerous mass, I am sad to say, could also cause bleeding as you describe if it ruptured. It's possible that a good physical exam would reveal the problem or she might need to be scoped to find either a polyp or mass. Then treatment options could be discussed.
I would love to be able to tell you something to give her over the counter for this problem, but given the possible serious nature of what's going on with her, there really isn't anything that is likely to help. She really needs to be seen by your vet.
I realize that my answer may not be what you want to hear but I would be doing a disservice to both you and Crystal if I were less than truthful and honest in my response to you. I hope you understand.
I also hope that this is helpful and that Crystal's problem will turn out to not be a serious one. Deb